An F2-
excimer laser has multiple closely-spaced spectral lines of interest around 157 nm, and one of the lines is selected by
wavelength selection
optics. The
wavelength selection
optics of a first preferred embodiment include a birefringent Brewster window enclosing the
laser gas volume of the
discharge chamber. The window preferably comprises MgF2 and is located at one end of the
discharge chamber. One line is selected of the two when the optical thickness of the window is selected in coordination with rotatably adjustable, orthogonal refractive indices of the window. The transmissivity of the window is dependent on the orthogonal refractive indices and the optical thickness of the window. The
wavelength selection
optics of a second preferred embodiment include are at least partially within the
laser active volume. In this way, line selection is performed in a manner which optimizes the combination of optical and
discharge efficiency,
resonator size and cost. The wavelength selection unit preferably includes a
prism having a front surface oriented at Brewster's angle and a back surface oriented to receive and reflect an ordinary refracted
ray travelling within the
prism at a right angle to the back surface. The back surface also preferably includes a highly reflective
coating to serve as the highly reflective surface of the
resonator. The wavelength selection unit preferably further comprises an adjustment component for adjusting the orientation of the
prism and for enclosing the other end of the housing, opposite the outcoupling end.